People with or at a high risk of cardiovascular disease who maintain a vegetarian diet for 6 months or longer show significant improvements in key risk factors, including cholesterol, glycemic control and body weight, a meta-analysis of randomized controlled trials shows.
“To the best of our knowledge, this meta-analysis is the first that generates evidence from randomized controlled trials to assess the association of vegetarian diets with outcomes in people affected by cardiovascular diseases,” report the authors. The study was published online July 25 in JAMA Network Open.
“The greatest improvements in hemoglobin A1c and low-density lipoprotein cholesterol (LDL-C) were observed in individuals with type 2 diabetes and people at high risk of cardiovascular disease, highlighting the potential protective and synergistic effects of vegetarian diets for the primary prevention of cardiovascular disease,” they say.
Poor diet is well-established as increasing the morbidity and mortality associated with cardiovascular disease; however, although data has linked vegetarian diets to cardiovascular disease prevention in the general population, research on the effectiveness of such diets in people at high risk of cardiovascular disease is lacking.
“To the best of our knowledge, no meta-analysis of randomized controlled trials has been conducted to investigate the association of vegetarian diets with outcomes among people with CVD — indeed, research here has primarily focused on observational studies,” writes Tian Wang, RD, and colleagues at the University of Sydney, New South Wales, Australia.
Greater Decreases in LDL-C, A1c, and Body Weight With Vegetarian Diets
For the meta-analysis, researchers identified 20 randomized controlled trials involving vegetarian diets that included 1878 adults with or at a high risk of cardiovascular disease and included measurements of LDL-C, A1c, or systolic blood pressure.
The studies were conducted in the United States, Asia, Europe, and New Zealand between 1990 and 2021. Sample sizes ranged from 12 to 291 participants.
The mean range age of participants was 28 to 64 years. Studies included patients with cardiovascular disease (four studies), diabetes (seven studies), and those with at least two cardiovascular risk factors (nine studies).
The mean duration of the dietary intervention was 25.4 weeks (range 2 to 24 months). The most commonly prescribed diets were vegan (plant-based foods only), lacto-ovo-vegetarian (excluded meat, poultry seafood, and dairy products, but allowed eggs), and lacto-vegetarian (same as previous but allowed dairy products).
Overall, those who consumed a vegetarian diet for an average of 6 months, versus comparison diets, had significantly greater decreases in LDL-C (6.6 mg/dL beyond the reduction achieved with standard therapy); A1c (0.24%); and body weight (3.4 kg), but the reduction in systolic blood pressure (0.1 mmHg) was not significantly greater.
Assessment of the overall certainty of evidence evaluated using the Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) tool showed a moderate level of evidence for reductions in LDL-C and A1c with the vegetarian diet.
Lacto-ovo vegetarian diets were associated with the greatest reduction in LDL-C (14.1 mg/dL); however, four out of the five trials restricted energy intake.
Of note, vegetarian diets were most effective for achieving glycemic control among people with type 2 diabetes and leading to improvements in weight among those at high risk of cardiovascular disease as well as those with type 2 diabetes.
The effects “suggest that vegetarian diets might have a synergistic [or at least nonantagonistic] use in potentiating the effects of optimal drug therapy in the prevention and treatment of a range of cardiometabolic diseases,” the authors write.
Althoug previous studies have shown similar improvements associated with a vegetarian diet, most studies did not stratify populations based on disease status, type of vegetarian diet, or comparison diet, the authors note.
The lack of improvement in systolic blood pressure is consistent with previous meta-analyses of vegetarian diets in general and suggests that salt intake may be the more important factor for those measures.
“[The meta-analysis] suggests that diet quality plays a major role in lowering blood pressure, independent of animal food consumption, as the DASH [Dietary Approaches to Stop Hypertension]…trial demonstrated,” the authors note.
Decreases in Medication Dose With Vegetarian Diet
Alhtough most patients were taking medications to manage hypertension, hyperglycemia, and/or dyslipidemia at trial enrollment in as many as eight of the studies, the vegetarian diet intervention resulted in a decrease in medication dose.
In fact, medication use could obscure the favorable effects of vegetarian diets, which could have a larger effect size, the authors speculate.
“This hypothesis is supported by two randomized controlled trials in our meta-analysis that required patients not to take medication that could influence cardiometabolic outcomes, [and] these studies significantly improved systolic blood pressure and LDL-C,” they write.
Not All Vegetarian Diets Are Healthy
Although there are numerous variations in vegetarian diets, ranging from vegan diets that eliminate all animal food to pesco-vegetarian diets that allow fish or seafood, most that are well-balanced can provide health benefits including lower saturated fat, L-carnitine, and choline (precursors of the atherogenic TMAO), and other benefits that might explain the improvements seen in the meta-analysis.
The diets may also be high in dietary fiber, mono- and polyunsaturated fatty acids, potassium, magnesium, and phytochemical, and have lower glycemic index scores.
Of note, 12 studies in the meta-analysis emphasized low-fat content, which the authors speculate may have contributed to the improvements observed in LDC-C.
Specifically, lacto-ovo vegetarian diets were associated with the greatest reduction in LDL-C (−14.1 mg/dL); however, four out of five of the trials restricted energy intake, which could have also played a role in improvements.
Importantly, not all vegetarian diets are healthy, and the authors caution about some that allow, for instance, deep-fried foods rich in trans-fatty acids and salt, such as tempura vegetables, potentially increasing the risk of type 2 diabetes and coronary heart disease.
They note that “more than one third of the studies included in our meta-analysis did not emphasize the importance of consuming minimally processed plant-based whole foods.”
Overall, however, the fact that the greatest improvements in A1c and LDL-C were seen in patients with type 2 diabetes and those at high risk of CVD “highlight[s] the potential protective and synergistic effects of vegetarian diets for the primary prevention of CVD.”
JAMA Netw Open. 2023;6:e2325658. Full text
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